In recent years, vehicles with an idle stop control mechanism, which automatically shuts down the engine while the vehicle stops, for example, at a red light, when predetermined shutdown conditions have been met, have been in practical use. The automatic transmission mounted on such a vehicle includes an electrically-operated oil pump, in addition to a mechanically-operated oil pump driven by the engine, to make a prompt start at the vehicle's next start. The hydraulic pressure generated by this electrically-operated oil pump is used to bring a frictional engagement starting element, which is engaged at a start of the vehicle to transmit power, into an engaged state in advance or into a ready-to-engage state while the engine is automatically shut down.
Examples of the ready-to-engage state include bringing the frictional engagement starting element into a slip state, and in the case of double-piston type frictional engagement starting element having an engagement piston and a clearance adjustment piston, bringing the frictional engagement starting element into a state in which the hydraulic pressure is supplied only to a hydraulic pressure chamber for the clearance adjustment piston.
With respect to such a hydraulic control unit for the automatic transmission, Patent Document 1 discloses a hydraulic pressure circuit 600 having a selector valve 620 at a path for supplying hydraulic pressure to a frictional engagement starting element 610, as illustrated in FIG. 13. The selector valve 620 switches the hydraulic pressure supply source to a mechanically-operated oil pump 606 (hereinafter referred to as a mechanical pump 606) or to an electrically-operated oil pump 616 (hereinafter referred to as an electric pump 616). In this hydraulic pressure circuit 600, the spool 622 of the selector valve 620 is movable between a first position at which the mechanical pump 606 is the hydraulic pressure supply source, and a second position at which the electric pump 616 is the hydraulic pressure supply source. Specifically, while the engine is in operation, the hydraulic pressure is input to a control port 630 from the mechanical pump 606, causing the spool 622 to be located at the first position (the position on the right side in FIG. 13). On the other hand, while the engine is shut down, the hydraulic pressure is not supplied to the control port 630 from the mechanical pump 616, and hence, the elastic force of a return spring 640 causes the spool 622 to be located at the second position (the position on the left side in FIG. 13).
Actuation of the electric pump 616 is started at the implementation of an automatic shutdown of the engine. At this start of actuation, a discharge line 650 of the electric pump 616 is blocked by the spool 622 that is still present at the first position. Here, a drain line 660 provided with an orifice 662 branches from a portion of the discharge line 650 between the electric pump 616 and the selector valve 620. This drain line 660 prevents the discharge oil from the electric pump 616 from being trapped in the discharge line 650. As a result, the motor 615 which drives the electric pump 616 is prevented from receiving an excessive load at the start of actuation of the electric pump 616. This leads to reduced energy loss due to heat generation, and reduced degradation of the durability. In addition, in the case where the electric pump 616 is of an automatic rotation control type, phenomena, such as step-outs, in which the rotation cannot be properly controlled, can be reduced.